Continuous process for the production of carboxylic acid esters of alkylene glycol monoalkyl ethers

ABSTRACT

The present invention provides an improved method for the preparation of carboxylic acid esters of alkylene glycol monoalkyl ethers by the acid catalyzed esterification of the monoalkyl ether with a carboxylic acid. In a preferred embodiment of the invention, the carboxylic acid and alcohol are reacted in a reactor/column and the resulting ester product is distilled into an overhead decanter/extractor as a single phase. A small amount of solvent, preferably a hydrocarbon is added to the mixture causing the resulting distillate to separate into two phases, one phase containing the desired product, the other containing primarily water. The process described is applicable to both batch and continuous operation and is not constrained by the difficulty of separating closely boiling azeotropes and results in substantially higher production rates than achieved by current processes.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on PCT application: PCT/US 97/19827.

FIELD OF THE INVENTION

The present invention relates generally to preparation of carboxylicacid esters and specifically to a method for preparing alkoxy alkylesters.

BACKGROUND OF THE INVENTION

The present invention is directed to preparation of carboxylic acidesters and specifically to a method for preparing alkoxy alkyl esters ofalkylene glycol monoalkyl ethers in a continuous mode. The reactionliberates water which in addition to unreacted reactants causesoperational and purification problems.

In continuous processes to produce esters by reaction of an alcohol anda carboxylic acid, the water of reaction is removed to increaseconversion. Typically, the reaction is carried out using a reactorcontaining a mixture of alcohol, carboxylic acid, ester, water, and anacid catalyst. The reactor is heated to obtain an equilibrium mixtureand the products distilled in a fractionating column. As product isdistilled alcohol and carboxylic acid are fed to the reactor. Withsimple esters such as ethyl acetate and butyl acetate the water isremoved as azeotropes with the ester and unconverted alcohol. Thedistillate separates into two liquid phases. The upper phase, referredto as the ‘oil phase’, contains mainly ester with a little alcohol andsome water. The lower phase, referred to as the ‘water phase’, containsmostly water with some ester and alcohol. The water phase is transferredto a distillation tower and the water discharged from the bottom of thetower as waste; the distillate is recycled. The oil phase is distilledin a purification tower to produce a base discharge product of pureester and a distillate which is recycled to the reactor. This processhas been optimized over the years to allow production of these simpleesters at high rates.

When it is attempted to esterify alkylene glycol monoalkyl ethers suchas 1-methoxy-2-propanol using this process it has been found to workpoorly, if at all. The distillate from the reactor column does notreadily separate into two phases, making it very difficult to remove thewater of reaction by the above process. The reason that phase separationdoes not occur is that the alcohol and ester are much more soluble inwater compared to simple ester. It has been found that it is possible tooperate the reactor/distillation tower in such a manner so as toseparate two closely boiling azeotropes (one richer in the alkyleneglycol monoalkyl ether and the other richer in the corresponding ester).Although this can be accomplished by operating the distillation tower ata high reflux to distillate ratio, operation of the distillation towerin this manner greatly decreases its capacity. This produces adistillate, which does separate into an oil phase and a water phase, butthe degree of separation is poor. Furthermore, the reactor/distillationtower must be operated at such a low rate to make the overall productioneconomically unfeasible.

Because of the solubility problem described above, the alkylene glycolmonoalkyl ether esters are usually manufactured by a process describedin European Patent Application 0119833 B1. A compound such as toluene isadded to the reactor and the water is removed by distillation as anazeotrope with toluene. This drives the reaction to completion. Theazeotrope separates into two phases; the water is removed as the waterphase and the oil phase is recycled to the reactor. In this process onlywater is distilled as the azeotrope, leaving ester, unreacted alcohol orcarboxylic acid, and catalyst in the reactor. This process requiresremoval of the catalyst from the product by neutralization or some othermeans prior to purification. Another drawback is that these processesare normally run in batches rather than in a continuous mode and resultin low raw material efficiencies and loss of catalyst. Moreover, estermade this way tends to have problems with acidity and stability.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the foregoingdifficulties. We have discovered that an alkylene glycol monoalkyl etherester, such as 1-methoxy-2-propyl acetate can be produced in acontinuous process in high yield, at high rate, with excellent productquality, and without catalyst loss. This is accomplished using water asthe azeotropic agent and distilling the product as an azeotrope ofwater, carboxylic acid ester, and some unreacted glycol ether alcoholfrom the reactor into an overhead decanter/extractor. This productresults in a single phase. A small amount of an inert solvent is fed tothe decanter/extractor causing the distillate to separate into an oilphase and a water phase. The oil phase contains primarily the solvent,ester, and a small amount of water and reacted alcohol. The water phasecontains primarily water and unreacted alcohol and some ester. Unreactedcarboxylic acid and catalyst remain in the reactor. This process is notconstrained by the difficulty in separating closely boiling azeotropesor higher boiling ester products and results in substantially higherproduction rates. Moreover, the carboxylic acid is not distilledoverhead and does not contaminate the product.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a preferred embodiment of the present invention andillustrates the esterification procedure employing a reactor/reboiler, adistillation tower and a decanter/separator.

DESCRIPTION OF THE INVENTION

The present invention provides a method to manufacture alkoxy alkylesters by the reaction of a carboxylic acid and an alcohol. In apreferred embodiment of the invention, the carboxylic acid and alcoholare reacted in a reactor/reboiler and the resulting ester product iswater azeotropically distilled into an overhead decanter/extractor as asingle phase. A small amount of an extraction solvent is added to themixture causing the resulting distillate to separate into two phases,one phase containing the desired product, the other containing primarilywater. The process described is not constrained by the difficulty ofseparating closely boiling azeotropes of reactants and products or byhigh boiling point ester products, and results in substantially higherproduction rates than achieved by current processes.

In accordance with this invention there is provided a method for thepreparation of carboxylic acid esters of alkylene glycol monoalkylethers comprising:

a) reacting a monocarboxylic or halogenated monocarboxylic acid havingfrom 1 to about 10 carbon atoms, with an alkylene glycol monoalkyl etherhaving the formula

 wherein

n=0-6;

R₁, R₂=H, CH₃—(CH₂)_(n)—; provided when n=6, R₁ or R₂=H, and

X=Cl, Br, F

in the presence of an acid catalyst; the X group may be located anywherein the chain;

b) distilling the mixture in a distillation tower, while using water toazeotrope the carboxylic acid ester and unreacted alkylene glycolmonoalkyl ether;

c) directing the distillate of (b) to an overhead extractor andcontacting with an effective amount of inert solvent (also referred toas phase separating agent) to enable formation of at least two liquidphases;

d) separating the resulting phases of the mixture (water phase and oilproduct) phase);

e) distilling the oil phase to recover (substantially pure)monocarboxylic acid ester product and inert solvent (for recycle); and,

(f) distilling the water phase (to remove water for waste disposal andalcohol and ester for recycle).

The process can be applied to a continuous or batch reaction set up. Itis preferably applied to a continuous reaction setup involving a reactorcolumn, a distillation tower and an overhead decanter/extractor.

Examples of C₁₋₁₀ acids include but are not limited to: acetic acid,formic acid, propionic acid, i-butyric acid, and n-butyric acid.Examples of glycol esters of the product of the process include but arenot limited to 1-ethoxy-2-ethyl acetate, 1methoxy-2-propyl acetate, and1-methoxy-2-propyl propionate. Examples of useful ethers include:2-ethoxyethanol, and 1-methoxy-2-propanol, and the like.

The reaction is catalyzed by an acid such as a mineral acid such asconcentrated sulfuric acid, hydrochloric acid, nitric acid and the like.Lewis acids such as boron trifluoride, antimony pentafluoride and thelike may also be employed. Organic sulfonic acids and halogenatedsulfonic acids such as methane, ethane and butane sulfonic acids,trifluoromethane sulfonic acid, trichloromethane sulfonic acid, o- orp-toluene sulfonic acid, benzene sulfonic acid and the like as well asstrongly acidic sulfonated aromatic ionic exchange resins andperfluoroalkane sulfonic acid resins are also useful. The acid catalystsare generally employed in concentrations of from about 0.01 to about 10wt %, preferably from about 0.1 to about 2.0 wt %, based on the totalreaction mixture, which concentrations may vary with the particular acidemployed.

The following paragraph explains the difference between the azeotropicprocesses currently employed and use of a phase separating agent(extraction agent) in our invention. When a hydrocarbon, for example, isused as an azeotropic agent in a process, it is added to or present inthe reactor/reboiler. A constant boiling mixture distills through thedistillation tower to produce a distillate containing the hydrocarbonand other components, in this case, primarily water. The desiredproduct, unreacted alcohol and/or carboxylic acid, and catalyst is leftin the reactor/reboiler. When a hydrocarbon is used as a phaseseparating agent (extraction agent) in the process of this invention, itis added to the overhead decanter/separator, causing the product toseparate into two phases. By operating this way no carboxylic acid isdistilled, greatly simplifying purification of the product.

Useful phase separating agents include those solvents which are inert,have compatible chemistry with the reaction components and cause thedesired product to separate into phases. Generally, any solvent havingthese characteristics and low water solubility is suitable. The solventmay be a linear, branched, aromatic, or cyclic hydrocarbon, an ester,ether, ketone, or fluoro chloro compound. Generally, those compoundshave from about 5 to 12 carbon atoms. Example of suitable solventsinclude, but are not limited to: pentane, cyclopentane, hexane,cyclohexane, toluene, benzene, xylene, olefinic hydrocarbons, butylacetate, propyl acetate, ethyl acetate, methyl t-butylether,diisopropylether, methyl ethyl ketone, methyl propyl ketone, methylbutyl ketone, and corresponding compounds, fluoro chloro hydrocarbons,chloroform, carbon tetrachloride, methylene chloride, and Freons®).Preferred phase separating solvents are C₅-C₁₂ hydrocarbons, especiallywhen employed under atmospheric conditions. Hydrocarbons greater thanC₁₂ are not preferred since generally, if the hydrocarbon has too high aboiling point, the hydrocarbon tends to go to the reactor, not to thedistillation tower overhead receiver/decanter. If the hydrocarbon hastoo low a boiling point, it is not practical to employ under atmosphericconditions. Olefinic hydrocarbons may be employed, but are not preferreddue to their tendency to polymerize in the reactor.

The phase separating solvent is employed in an effective amount toenable formation of two liquid phases within the decanter/extractortemperature range of operation. Suitable amounts include from about 5 toabout 70 wt %, preferably 10 to about 50 wt %, and, most preferablyabout 20 to about 40 wt %. Too little phase separating agent will notcause phasing, and too much will require excessive equipment size andenergy consumption to process.

The acid catalyzed esterification reaction may be carried out in anysuitable reactor, said reactor having means for mixing of reactants,regulating temperature of the reaction, and means for separating thedesired ester product from the unreacted components, and water which isgenerated during reaction. In a preferred embodiment, employed, inaddition to that mentioned previously, is a distillation column, acondenser and a phase separator or decanter/extractor for removing thesolvent(containing product)-water phases, and a means for returning thesolvent and the water to a distillation column

A general procedure for carrying out the reaction is to charge theglycol ether, carboxylic acid and acid catalyst into a reaction vesselor reaction column. Heat the mixture and maintain at the desiredreaction temperature for an appropriate period of time, and thentransferring the distilled product mixture to an overhead phaseseparator. Contacting the mixture in the phase separator with a phaseseparating solvent and allowing the phases to separate. The productisolation process then proceeds by separating out the resulting phasesof the mixture (water phase and oil (containing product) phase),distilling the phases, and recovering the desired monocarboxylic acidester of interest.

Referring to FIG. 1, illustrated is a reactor/reboiler wherein reactantsare contacted together and mixed throughly employing standard reactionengineering methods. If running in a continuous mode, the feed rate ofreactants is adjusted to maintain a suitable residence time at reactiontemperature. The mixture is directed to the base of a distillation towerwherein ester product, water, and unreacted reactants are distilled. Thedistilled product stream containing the desired ester product is thendirected to a decanter/separator wherein phase separating agent is addedto the mixture. Generally within a short amount of time, typicallyminutes, after adding the phase separating agent, the mixture results inat least two liquid phases. The oil or product phase is separated fromthe water phase. The product phase is then distilled to achieve a higherlevel of purity. The water phase containing mostly water, some ester,and some unreacted alcohol is directed to an organic recovery tower.

General reaction conditions for the inventive esterification include atemperature range in the reactor/column of about 80 to about 160° C., apressure of about 0.1 to 10 atm and a reactor residence time of about0.3 to about 5 hrs. The three parameters can be adjusted to optimize theprocess, and will be different for each ester produced. For reasons ofeconomy, the preferred conditions are operation near 1 atm pressure andwith a reactor residence time of about 0.5-2 hrs.

Although the method of the present invention is directed to theproduction of alkylene glycol monoalkyl ether esters, the procedure isapplicable to general esterification reactions. Those of skill in theart will also recognize that the present method is broadly applicable tothe preparation of other esters such as propylene glycol monobutylacetate, dipropylene glycol monoctyl butyrate, ethylene glycolmonoethylformate, etc., using the appropriate glycol ether andmonocarboxylic acid.

The following examples are intended for illustrative purposes and arenot intended to limit the scope of the present invention.

EXAMPLES Example 1

An apparatus was assembled having a 30 tray, 2″ diameter Oldershawdistillation column, a reflux condenser, an overhead receiver(decanter), and a reboiler/reactor. Pumps were used to feed freshmaterial to the reboiler and cyclohexane to the overhead receiver. Tothe reboiler/reactor was added 62.8 grams of 1-methoxy-2-propyl acetate,66.0 grams of 1-methoxy-2-propyl alcohol, 132.1 grams of glacial aceticacid, 67.1 grams of water, and 17.9 grams of methanesulfonic acidcatalyst The distillation column was operated at atmospheric pressure ata reflux to distillate ratio of 1.0. Fresh material, having acomposition of 44.0 wt % 1-methoxy-2-propyl alcohol, 14.0 wt % glacialacetic acid, and 42.0 wt % water, was fed to the reboiler at a rate of5.28 grams/min. Cyclohexane was fed to the overhead receiver at a rateof 1.07 grams/min. During operation the temperature in the reboilerremained at 112° C. and the temperature at the top tray of thedistillation column at 94° C.; this ensured that little or nocyclohexane was present in the distillation tower or reactor/reboiler.The total product rate from the overhead decanter was 6.36 grams/min,including the cyclohexane feed. The condensed distillate immediatelyseparated into an oil phase containing primarily cyclohexane and1-methoxy-2-propyl acetate and an aqueous phase containing mostly waterwith some 1-methoxy-2-propyl alcohol and 1-methoxy-2-propyl acetate.Only the aqueous phase was refluxed to the distillation tower. Theseoperating conditions were maintained for five hours.

The composition of the distillate prior to addition of cyclohexane wasdetermined to be 33.1 wt % 1-methoxy-2-propyl acetate, 21.2 wt %1-methoxy-2-propyl alcohol, and 45.7 wt % water. It was determined fromprior experimentation that under these reaction conditions this mixturewill not separate into two phases. After addition of cyclohexane andseparation of the phases, the oil phase contained 44.1 wt % cyclohexane,46.6 wt % 1-methoxy-2-propylacetate, 7.7 wt % 1-methoxy-2-propylalcohol, and 1.6 wt % water. Phasing was also observed with about 10 wt% of cyclohexane.

Example 2

Following the procedure in Example 1, the distillation column wasoperated at a reflux to distillate ratio of 0.68. Fresh material, havinga composition of 47.7 wt % 1-methoxy-2-propyl alcohol, 18.2 wt % glacialacetic acid, and 34.0 wt % water was fed to the reboiler at a rate of5.85 grams/min. Cyclohexane was fed to the overhead receiver at a rateof 2.01 grams/min. During operation the temperature of the reboilerremained at ˜115° C. and the temperature at the top of the distillationtower at ˜93° C. The total product rate from the overhead receiver was7.78 grams/min including the cyclohexane feed. This separated into anoil phase and an aqueous phase as in the previous example. Theseoperating conditions were maintained for five hours.

The composition of the distillate prior to addition of cyclohexane tothe overhead receiver was 32.5 wt % 1-methoxy-2-propyl acetate, 19.3 wt% 1-methoxy-2-propyl alcohol, and 48.2 wt % water. It was determinedfrom prior experimentation that this mixture will not separate into twophases.

Composition of product phase after addition of cyclohexane andseparation of the phases contained 50.8 wt % cyclohexane, 43.5 wt %1-methoxy-2-propyl acetate, 5.7 wt % 1-methoxy-2-propyl alcohol, and 0.0wt % water.

Comparative Example

Many experiments without addition of cyclohexane showed that phaseseparation could not be achieved in the overhead receiver unless thereflux to distillate ratio in the tower was at least 3.0, and preferablygreater than 5.0 using a 30 tray column. At these operating conditions,however, the feed to the reboiler could only be sustained at a rate of˜1.0 g/min. In a practical sense, not even this rate would be possiblewhen recycle of unreacted 1-methoxy-2-propyl alcohol and operation ofthe water removal column is considered.

The examples show that the desired ester product can be removed at arate >5 times that when not using a phasing agent. They also show that adistillable product is obtained containing the desired ester, that isfree of carboxylic acid reactant. They illustrate a practical processthat does not require neutralization of acid catalyst to recover pureproduct.

What is claimed is:
 1. A method for the preparation of a carboxylic acidester of an alkylene glycol monoalkyl ether comprising a) reacting in areaction column a monocarboxylic or halogenated monocarboxylic acidhaving from 1 to 10 carbon atoms with an alkylene glycol monoalkyl etherhaving the formula

 wherein n=0-6; R₁, R₂=H, CH₃—(CH₂)_(n)—; provided when n=6, R₁ or R₂=H,and X=Cl, Br, F in the presence of an acid catalyst; b) distilling themixture in a distillation column, while using the water of reaction toazeotrope the carboxylic acid ester and unreacted alkylene glycolmonoalkyl ether; c) transferring the distillate of (b) to an overheadphase extractor/separator and contacting with an effective amount ofinert solvent to enable formation of at least two phases; d) separatingout the resulting phases of the mixture so as to form a water phase andan oil (product) phase; and, e) distilling the oil phase to recovermonocarboxylic acid ester product and inert solvent.
 2. The method ofclaim 1 wherein the acid catalyst is selected from the group consistingof concentrated sulfuric acid, hydrochloric acid, nitric acid, borontrifluoride, antimony pentafluoride, methane sulfonic acid, ethanesulfonic acid, butane sulfonic acid, trifluoromethane sulfonic acid,trichloromethane sulfonic acid, o-toluene sulfonic acid, p-toluenesulfonic acid, benzene sulfonic acid, sulfonated aromatic ionic exchangeresins and perfluoroalkane sulfonic acid resins.
 3. The method of claim1 wherein the ether is selected from the group consisting of1-methoxy-2-propanol and 2-ethoxyethanol.
 4. The method of claim 1wherein the inert solvent is linear, branched, aromatic, cycloaromatichydrocarbon, ester, ether, ketone, and fluoro chloro compounds.
 5. Themethod of claim 4 wherein the solvent is selected from the groupconsisting of a C₅₋₁₂ hydrocarbon, pentane, cyclopentane, hexane,cyclohexane, toluene, benzene, xylene; and butyl acetate, propylacetate, ethyl acetate, MTBE, diisopropylethers, methyl ethyl ketone,methyl propyl ketone, methyl butyl ketone, methylene chloride, carbontetrachloride, and any corresponding branched compounds.
 6. The methodof claim 5 wherein the solvent is employed at about 5-70 wt %.
 7. Themethod of claim 6 wherein the solvent is employed at about 10-50 wt %.8. The method of claim 7 wherein the solvent is employed at about 20-40wt %.